which way of bloggin is better for seo

I made a website to my cousin which is a static site with just few static pages.

i want to add a blog capability to her site so that she can write things when she wants, to have good quality content on her site as she is an expert in her field.

which way is better for seo: adding a link called blog to her existing static site, and adding a wordpress site in that blog link, or, make the whole site wordpress and then add back the existing static pages?

Do the whole site in wordpress, it takes care of all the mechanics for you so unless your quite skilled in writing html then wordpress will do a far better job for you.
Use pages for all the static stuff and so it acts like a conventional site. once you have the static pages in then allow her to start doing posts.